In recent years, optical elements such as lenses and prisms are required to be thinner and support various uses, and need to be precisely and uniformly molded into a unique shape. Particularly, high uniformity is required with respect to the optical surface shape precision and the inner birefringence. In addition, product lines, which are conventionally made of glass in a mainstream, are required to change into plastic optical elements (plastic lenses, plastic mirrors, etc.) to meet the requirements of cost reduction.
To precisely mold a plastic optical element into a desired shape by injection molding, in the process of filling a mold cavity with molten resin, uniform resin pressure and resin temperature are desired in the cavity to uniformly spread the resin in the mold. Further, the resin pressure needs to be kept being applied until the resin is fully solidified. Besides, regarding molded products in a complex shape with an uneven thickness, the speed of cooling solidification varies depending on the part, which results in the occurrence of internal stress. It is known that a molding method as described above is used to obtain a plastic optical element with accuracy by injection molding. Although improving the shape transfer performance for a resin mold surface, this method allows easy generation of birefringence due to the pressure-holding process and the internal strain.
Meanwhile, to reduce the birefringence, injection molding has to be performed with low resin pressure in the cavity (low-pressure injection molding). However, in the low-pressure injection molding, it is not possible to compensate for the volume of cooling shrinkage of resin filled in the mold. Accordingly, sink marks are likely to occur anywhere including molded articles, and the increase in the amount of shrinkage causes the optical surface to be obtained to separate from the mold transfer surface prior to being fully cured, resulting in deterioration in the performance of molding and transfer of the optical surface shape. Hereinafter, this minute transfer failure of the optical surface is defined and referred to as “surface division” to be distinguished from a general sink mark that can be evaluated from the external appearance. The surface division is explained below based on photographs of interference fringes (see FIGS. 14A, 14B) obtained by a laser interferometer. Comparing the interference fringes regarding the presence of a surface division, if there is a surface division (see FIG. 14B), it can be clearly seen that a boundary is generated due to the difference in transfer condition in the optical surface, and accordingly the surface is divided. FIG. 14A illustrates interference fringes when there is no surface division. As illustrated in FIG. 14A, if there is no surface division, a boundary is not caused by the difference in transfer condition in the optical surface, and thus no surface division occurs. Here, the “surface division” is defined as a minute transfer failure of the optical surface at a level where the presence or absence of the failure can be determined by interference fringes. FIG. 14C is a diagram illustrating shapes of central cross-sections in parallel to the longitudinal direction of photographs obtained by analyzing the interference fringes illustrated in FIGS. 14A and 14B. The solid line indicates a cross-section of FIG. 14A and is seen to form a continuous and loose concave of the optical surface. The dotted line indicates a cross-section of FIG. 14B, and it can be seen that a surface division occurs on the optical surface at a part where the line is discontinuous.
There has been known a plastic mold article molded by low pressure injection molding as a technology for reducing the birefringence of an optical element obtained by injection molding as small as possible, and also improving the necessary accuracy in the molding transfer of the optical surface. The plastic article includes a rib (projection) arranged in a boundary between a non-optical surface and an optical surface, an incomplete transfer face having a concave shape, which is arranged on the same face as where the rib is located and formed by incomplete transfer of the shape of a cavity of the mold, and an incomplete transfer face having a convex shape arranged at least one face other than the transfer face (see Patent Document 1). According to the above technology, sink area has less influence on the surface precision of the optical surface. Therefore, it is possible to improve the shape precision of the transfer surface without causing shrinkage and a surface division on the transfer surface.